1. Field of the Invention
The invention relates to an aperture plate for lithography systems, and more particularly, to an aperture plate which can improve the normalized image log slope (NILS) of lithography systems to avoid the problem of forbidden pitch.
2. Description of the Prior Art
The lithographic process is a very important step in semiconductor fabrication that transfers the layout of a designed integrated circuit onto a semiconductor wafer. Lithographic and development technologies are used to proportionally transfer the circuit design pattern onto a photoresist layer on the surface of the semiconductor wafer. As the complexity and the integration of semiconductor circuits increases, the size of the circuit design pattern on the photoresist layer decreases. However, the critical dimensions (CDs) of the pattern on the photoresist layer are limited by the resolution limit of the optical exposure tool. As a result, optical proximity effects easily occur in the lithographic process during the formation of highly integrated circuit design patterns on a semiconductor wafer resulting from overexposure or underexposure, which causes a loss of resolution. In other words, optical proximity effects cause a difference between the pattern transferred onto the photoresist layer and the original design pattern.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a lithography system 10 according to the prior art. The lithography system 10 comprises a scanner system 12 and a semiconductor wafer 24. The scanner system 12 includes a light source 14 positioned near a top region of the scanner system 12, a first lens 16, an aperture plate 18, a second lens 20, and a photomask 22. Light beams from the light source 14 pass through the first lens 16, the aperture plate 18, the second lens 20, and the photomask 22 to the die region 26 to define a pattern of the photomask 22 on the photoresist layer on the wafer 24.
The aperture plate 18 is an application of the off-axis illumination (OAI) method, which has a light-intersecting region on the middle area of the aperture plate 18 for blocking vertical incident light to increase the resolution and depth of focus (DOF) of the lithography process. In order to increase the integration of semiconductor products unceasingly and keep the accuracy at the same time, manufactures have undertaken research and development to refine the aperture pattern of the aperture plate 18 so as to improve the transferring performance through a common light source, such as the light with a wavelength of 248 nm, and the equipment on hand. Among the several kinds of aperture patterns, Quadrupole 90 is a four-pole aperture pattern, which has an included angle of 90 degrees with the horizontal axis. In contrast to other aperture patterns, such as an annular aperture, Quadrupole 90 can improve the array CD of the lithography process. However, when the design pattern of the photomask is more and more delicate, the lithography system using the Quadrupole 90 aperture plate encounters the problem of forbidden pitch on the condition of specific pitches, which means the NILS cannot match the lowest requirement of the lithography system, resulting in defects of transferring performance.